In recent years, a resolution of TFT (Thin Film Transistor) liquid crystal displays is getting finer and finer. In this environment surrounding the TFT liquid crystal display, the available charge time for pixels, which can be regarded as a kind of a capacitor, is getting shorter. Such conditions often lead to situations where a sufficient amount of charge necessary for intended gray scale display cannot be obtained. The liquid crystal display also has a drawback that response speed is slow.
One method to solve such problems is to supply a gate driver output not only to the TFTs of a line being charged but also to the TFTs of the line two lines ahead of the target line, so as to pre-charge a liquid crystal pixel of the line two lines ahead of the target line prior to actual charging (hereinafter referred to as “gate-jumping two pulse driving”), as disclosed in Japanese Laid-Open Patent Publication 134293/1985 (Tokukaisho 60-134293, published on Jul. 17, 1985). Further, in Japanese Laid-Open Patent Publication 232651/1998 (Tokukaihei 10-232651, published on Sep. 2, 1998), it is proposed to delay a start timing of transmitting a gate output according to rounding of a source driver output in the “gate-jumping two pulse driving.”
Applying this method, it is possible to compensate for inadequate charging resulting from the improved resolution of the TFT liquid crystal display and the higher driving frequency. A response speed of the TFT liquid crystal display can also be improved.
However, in the “gate-jumping two pulse driving,” the conventional structures mentioned above have a problem of uneven gray scale display due to overcharging. A specific example of this is explained below.
As shown in FIG. 8, in the conventional “gate-jumping two pulse driving,” exactly the same charge is supplied to the actually charged line and pre-charged line, when pre-charging is carried out on TFT liquid crystal pixels by jumping one line from the actually charged line. This is because an actually charged pixel and a pre-charged pixel both lie on the same source line, which makes it impossible to charge a pixel of the actually charged line with the data of an intended gray scale level, and at the same time pre-charge a pixel of the pre-charged line on the same source line with data of a similar gray scale level.
When the gray scale display intended for the actually charged line has a gray scale level that requires a higher charge voltage than a gray scale level of the gray scale display intended for the pre-charged line two lines before the actually charged line, the pre-charging inevitably overcharges a liquid crystal pixel of the pre-charged line more than the intended level, as illustrated in FIG. 9. Note that, in a gate one pulse mode in which each pixel is charged only by actual charging without carrying out pre-charging, each line can be charged according to an intended gray scale level, and no overcharge occur. As such, normal gray scale level display is possible.
In the “gate-jumping two pulse driving,” the overcharged line by the pre-charging needs to attain an intended charge level in the actual charge period by discharging the excess charge gained by the pre-charging. However, in reality, due to the ON characteristics of the TFT, it is not always possible to discharge the excess charge. As a result, the resulting gray scale display becomes look nearly the same as the one displayed in the previous line two lines before, i.e., a gray scale level completely different from the intended level is displayed.
Because of these problems, the “gate-jumping two pulse driving” has not come into practical use, though it is effective in improving a resolution and response speed of the TFT liquid crystal display in TFT liquid crystal driving.